1. Field of the Invention
This invention relates to a semiconductor switch circuit including a PNPN switch which can be used as a switching element for a control device and is capable of making the gate turn-off operation with small control power.
2. Description of the Prior Art
A PNPN switch with a gate terminal has the advantages that a large current can be controlled by a small gate drive current, that it has a self-holding ability and that a high breakdown voltage is obtained in both directions. Thus, it is used with various control circuits. There are two methods of cutting off this PNPN switch having the self-holding ability. One is to cut off the main current, and the other to apply a negative current to the gate thereof. The former method is employed for the commonly-used PNPN switches, and one of the PNPN switches designed to facilitate the introduction of the latter method is a gate turn-off thyristor (hereinafter referred to as the GTOSCR). The turn-off gain G.sub.OFF (the ratio between the load current and the current required for the gate turn-off operation of the GTOSCR) of the GTOSCR is expressed as .alpha..sub.2 /(.alpha..sub.1 + .alpha..sub.2 - 1) for the P gate, where .alpha..sub.1 is the current gain of the PNP portion of the four-layered PNPN structure and .alpha..sub.2 the current gain of the NPN portion thereof. In order to increase the value of G.sub.OFF, it is necessary to make the value of .alpha..sub.2 as large as possible and the value of (.alpha..sub.1 + .alpha..sub.2) slightly larger than 1. In the GTOSCR, the current gain of the PNP portion must be reduced as much as possible, while the current gain of the NPN portion must be increased as much as possible. For this purpose, it is necessary to selectively diffuse gold only into the N base layer. If the lateral resistance of the gate layer (P base layer) is large, on the other hand, the voltage drop across this resistance causes a forward bias to be automatically applied between the gate and the cathode of the GTOSCR. To cut off the large load current under this condition, a very large negative current must be applied to the gate and, therefore, the large control power is required. The conventional method of minimizing such a resistance is to construct the gate electrode in such a special shape as a ring or comb.
Another disadvantage of the conventional PNPN switch is that a turn-off time is delayed because the PNP and NPN transistor portions thereof are generally deeply saturated during the ON state of the PNPN switch.